1. Field of the Invention
The invention relates to a process for the preparation of aqueous sodium methioninate solutions having a low sodium carbonate content from the crude hydrolysis mixtures obtained in the hydrolysis of 5-(.beta.-methylmercaptoethyl)-hydantoin with from 1.1 to 6 equivalents of sodium hydroxide and/or sodium carbonate, by separating off sodium carbonate monohydrate while heating, and also relates to granulates subsequently prepared from those solutions by various processes.
2. Background Information
Methionine and aqueous solutions of methionine salts, especially of sodium methioninate (DE 31 05 009 C), but also synthetic substances such as the methionine-hydroxy analogue (MHA), are used worldwide as feed additives for rearing poultry, swine and other useful animals and are of benefit mainly to the production of animal protein.
According to requirements, solid or liquid forms are preferably employed.
The sodium methioninate solution that is available on the market has a concentration of 40 wt. % methionine and, in contrast to MHA, corresponds to solid methionine, when compared on an equimolar basis, as regards its biological valency. Several methods come into consideration for the preparation of such sodium methioninate solutions, for example
1. Dissolution of isolated methionine in sodium hydroxide solution.
2. Alkaline hydrolysis of 5-(.beta.-methylmercaptoethyl)-hydantoin with NaOH and/or Na.sub.2 CO.sub.3, or with a mixture of NaOH/Na.sub.2 CO.sub.3 /NaHCO.sub.3.
3. Alkaline hydrolysis of methionine amide.
Although process 1 yields the purest form of product, it is, however, more expensive in comparison with production of the solid because it involves an additional process step, and it is, therefore, less economical than the preparation of methionine itself. Processes 2 and 3, on the other hand, start at an earlier stage in the preparation of methionine and hence achieve a relieving according to capacity of the solids portion of the DL-methionine preparation.
The preparation of 5-(.beta.-methylmercaptoethyl)-hydantoin takes place in a known manner by direct synthesis from the usual starting materials methylmercaptopropionaldehyde (MMP) and hydrocyanic acid in the presence of ammonia and carbon dioxide. Methionine amide is prepared in a known manner by hydrolysis of methionine nitrile, which in turn is obtained by direct synthesis from the usual starting materials MMP, hydrocyanic acid or ammonium cyanide and ammonia.
The hydrolysis solutions obtained according to process 2 contain relatively large amounts of sodium carbonate, which must be separated off.
To that end, the hydrolysis mixture is concentrated according to DE-OS 31 04 997 to a sodium methioninate content of from 40 to 65 wt. % and is then cooled to room temperature or below, and the sodium carbonate decahydrate that then forms is separated off. However, it is frequently obtained in a form which can be filtered only with difficulty.
According to U.S. Pat. No. 4,931,987, the procedure is reversed. The sodium carbonate is first allowed to crystallise out, and then the resulting solutions are concentrated.
DE-OS 31 05 009 discloses a process in which methanol or ethanol is added before the separation.
Depending on special requirements, it can be expedient to use a solid or liquid form of the animal feed supplement.
The decision regarding the form of administration is dependent, inter alia, on the available mixing tools and on the special preferences of each particular operator.
In the preparation of the mixed feed, the various feeds and additives are initially in the form of individual components which, according to their properties, are prepared, for example, by milling, coarse grinding, drying or purifying. If the individual components have the necessary properties, the actual mixing operation is carried out in a suitable mixing unit. The individual batches for mixing differ according to the size of the unit. In the supplementation of mixed feeds, the essential amino acid methionine is used in concentrations of the order of from 0.01 to 1.0 wt. %. Those amounts are added directly to the mixed feed by means of appropriate weighing and metering systems.
In DE-OS 31 05 009 it is described that aqueous solutions of sodium or potassium methioninate have the same methionine activity as solid methionine when used as a feed additive.
A methionine salt-based granulate is known from DE-197 07 380 of Feb. 15, 1997.